A circulator has generally been employed in microwave communications systems, and it has been desired to employ an optical circulator also in communications systems using optical fibers. In communications systems using optical fibers, however, the plane of polarization is disturbed when the light propagates through the optical fibers, making it difficult to maintain a predetermined plane of polarization. The microwave circulator has been based upon a prerequisite that the plane of polarization of the electromagnetic waves be maintained constant. The optical circulator, however, cannot be formed with a construction similar to that of the microwave circulator.